Dynamic random access memories (DRAMs) gradually become very important electronic devices. DRAMs generally include a great number of memory cells integrated in array regions for storing data. DRAMs further include peripheral regions for peripheral control circuits. Each of memory cells and the peripheral control circuits usually includes metal-oxide-semiconductor (MOS) transistors and other electronic devices such as series capacitors. In general, capacitors electrically connect with bit lines while control gates of MOS transistors electrically connect with word lines. In such an arrangement, each of the memory cells can be addressed. In order to electrically interconnect these electronic devices, a plurality of contact windows are formed between various layers, and then subsequently filled with conductive materials to serve as a part of interconnections among these electronic devices so as to achieve the functionality of the MOS transistors.
As the DRAM process advances to the scale of 100 nanometer, a plug is usually formed by forming a nitride spacer (e.g. silicon nitride spacer) on the sidewall of the contact window and followed by filling the contact window with undoped polysilicon to form a sacrificial plug. However, when the undoped polysilicon is removed by etching, the height of the spacer is difficult to control because the selectivity of the undoped polysilicon with respect to the nitride spacer is relatively low, that adversely increases the coupling capacitance between the word line and the bit line. Furthermore, as the distance between contact windows continuously decreases due to the shrinkage of the devices, the aspect ratio of the contact window gradually increases. As a result, the plugs disposed within the contact windows will crack easily such that the integrity and the reliability of the semiconductor device are unable to maintain. Therefore, it would be desirable to provide an improved method to eliminate the drawbacks of the conventional semiconductor devices.